The invention relates to a method for controlling a reluctance machine, in particular a switched reluctance machine.
Conventional drive systems, in particular for electrical vehicles, are frequently operated in a torque-regulated mode. However, it is advantageous to be able to operate such a drive system as an accelerator pedal controlled torque source, with a linear relationship between the control variable and the output variable being desirable.
In known, conventional drive systems, this requirement can be achieved by controlling the magnetic flux and/or the current. One precondition for this is that the flux and current are independent of one another, and can be influenced separately from one another.
This precondition is not satisfied in the case of reluctance machines since, owing to the single-sided excitation, the flux and current are fundamentally coupled to one another. A further exacerbating factor is that the coupling between the flux and current is generally not linear. No clear electrical signal is known from the prior art from which it would be possible to derive an instantaneous or average torque with sufficient accuracy.
The torque depends on the current waveform and its position with respect to the rotor rotation angle which, for its part, is influenced by being sensitive to the switching angles. Admittedly, values for the switch-on and commutation angles can be obtained from empirically obtained information about the switching angles, and these values are preferably stored in a memory and are available during operation. Although this allows a proportionality to be established between the nominal value and the actual value of the torque, it is not possible to establish a linear relationship. For this reason, rotation-speed-regulated reluctance drive systems can admittedly be produced. However, at the moment, there are few indications of production of a sufficiently accurate torque-regulated reluctance drive system.
Reluctance machines are particularly suitable for traction drive systems, in addition to conventional electrical machines, since they do not produce any electrical heat in the rotor and water cooling can be implemented in the stator allowing good utilization since, owing to the intrinsic characteristics of the magnetic circuit, they allow high torques even at low rotation speeds. Furthermore, they have very good partial-load efficiencies in the lower rotation-speed range.
The simple design of the machine leads to production advantages over conventional machines. The rotor has a small rotating mass, and the converter circuit has inherent resistance to short circuits.
The dissertation by A. Nickel at the Faculty of Electrical Engineering at the German Federal Armed Forces University, Munich, May 1998 has already proposed a torque-controlled, switched reluctance machine, in which a highly loaded machine is modelled and in which the simulation parameters obtained from this allow the winding currents, and the winding torque profiles which are dependent on them, to be calculated in advance with sufficient accuracy both during pulsed operation and during block operation. From this, the control parameters can be optimized for a low electrical heat level as a function of the mean value of the internal torque of the m windings, and can be calculated iteratively for the entire operation range.
The position of the switching angles in the torque nominal value rotor rotation angle plane is approximated by using parabolic functions, and that of the nominal current values is approximated by using root functions. The control coefficients obtained in this way are used for real-time calculation of the control parameters during operation. The switching angles calculated in this way are output to a converter, and the nominal current value is output to an analogue current regulation device with a two-point regulator.
In this case, during motor and generator operation, alternating clocking of the voltage is used in each case in pulsed operation. All three control parameters, the switch-on angle, the commutation angle and the winding nominal current value, are required in this case. In block operation, the control system primarily uses the two switching angles during motor operation. The nominal current value is used as a control variable only in the event of an overvoltage, in order to prevent the winding current from rising in an uncontrolled manner. During generator operation on the other hand, the control system operates with the switch-on angle and the nominal current value in block operation, and the commutation angle becomes less important.
The object of the invention is to improve a method for controlling a switched reluctance motor of the type described above.
This object is achieved by the features of the independent claim. Further advantages and refinements of the invention are evident from the other claims and from the description.
According to the invention, the commutation angle for each rotation speed is selected such that it at least remains constant, or increases, starting from a maximum value at a maximum torque, as the torque values decrease. A winding current preset nominal value for the windings of the reluctance motor is then sent as a control parameter to a current regulation device. The particular advantage is that, to obtain the commutation angle at a given rotation speed, it is either possible just to use a constant which is independent of the torque, rather than having to read its torque-dependent values from tables which consume memory space and commutation time, or else it is possible to use the already known or calculated path of the switch-on parabola. Nevertheless, the motor torque utilization is good.
It is advantageous that, at least in motor operation, the commutation angle at least remains constant or increases, starting from a maximum value with a maximum torque, as the torque values decrease. It is also advantageous, both in generator operation and in motor operation, to select the commutation angle for each rotation speed to be at least constant or increasing, starting from a maximum value at a maximum torque, as the torque values decrease.
The advantage with both measures is that both memory space and computation time can be saved in the control system. This makes it possible to calculate the control parameters in real time with sufficient accuracy and with a small angular error.
A further preferred embodiment of the invention is to use a constant commutation angle, which is independent of the nominal torque value, for each rotation speed. If the torque utilization is good, computation time and memory space are saved in a control system. It is particularly preferable, for a given rotation speed, to select the maximum commutation angle at the maximum torque to be constant.
A further preferred embodiment of the invention is to form the commutation angle using the relationship xcex3K(n)=c1+xcex3A(n), where xcex3A(n) is the path of the switch-on parabola as a function of the torque. It is particularly advantageous in this case that no additional computation complexity is required even though a torque-dependent variable is now being used for the commutation angle, since the switch-on parabola path has already been calculated. The torque utilization of the motor is further improved by the greater commutation angles at low torques.
It is advantageous to monitor for a maximum commutation angle, and in particular this reliably prevents a maximum commutation angle of more than 370xc2x0. Monitoring of the commutation angle is preferably implemented in machine control software.